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Basic Ditters have four alarms, one on the way up, and three on the way down. In the plane ride up, skydivers need to know when it is 1500 feet so they can take off their seat belts in case there is an emergency and they need to exit promptly. Furthermore, the door will open in summer time for fresh cold air and allow low altitude skydivers to jump. That has to be done above 1500 feet and anyone with a parachute or attached to a tandem instructor need to take off seat belt prior to opening the door. The first free fall alarm tells the skydiver that social time is over and to quickly move away from other skydivers. The next alarm happens after the previous alarm separation to indicate it is parachute opening time. The third free fall alarm is the strongest one. You will not hear it if you are under a parachute, but you will if you are about to make the evening news. With low altitude and high speed, this alarm is the last call for immediate actions.
You can get yourself a Ditter for $238 here:
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Found a $4 Arduino Pro Mini at Micro Center. Used one of my previous projects to program it since I'm too cheap to buy FTDI.
I'm also too cheap to buy 3D Printer, and Arduino Pro Mini fits perfectly in a dental floss case with room for transistor, buzzer, switch, and batteries. Did I forget anything?
Yeah, you can get BMP280 for a buck on eBay delivered to your home.
Not a big fan of pre-soldered pins? Break them off except for 11, 12, and 13 as I did in my previous project to make it easier to update sketches. Don't forget to solder a jumper to RST pin.
Pins A4 and A5 are the most challenging to solder. Solder the transistor directly on the buzzer. The black electric tap contains the batteries.
Originally, I had the buzzer pointed to the side, but it was not loud enough when installed in the helmet.
Had to rotate the buzzer 90 degrees and make another hole in the front of the case.
Since it will be too noisy to demonstrate in a real skydive, I decided to show how Abby perform in an elevator. Couldn't find any 13,000 feet elevator in Houston, Texas so I settled for this one. I just had to adjust altitudes on Abby as shown in code comments.